Refrigerator with a phase change material as a thermal store

ABSTRACT

A refrigerator having a thermal store comprising a phase change material is disclosed. The refrigerator has a cooling chamber for containing an object to be cooled, and a vapor compression refrigeration system including a first evaporator for cooling the cooling chamber and a second evaporator for cooling the phase change material. A valve is provided to control the flow of refrigerant to the first and second evaporators depending on the cooling load on the refrigerator. When the refrigerator is subject to a relatively low cooling load, refrigerant flows to the second evaporator to cool the phase change material and, when the refrigerator is subject to a relatively high cooling load, refrigerant flows to the first evaporator such that increased cooling is provided to the cooling chamber by the first evaporator and the phase change material.

The present invention relates to a refrigerator which uses a phasechange material as a thermal store. The invention is thought to be ofparticular relevance to commercial bottle coolers as found in bars andpubs because they experience relatively high peak cooling loads.

Refrigerated display cabinets of which bottle coolers are an example,are used extensively in entertainment venues to store and cool beveragesfor sale to customers. They are usually provided with a transparent orsee-through door so that the beverages inside can be displayed tocustomers.

Bottle coolers experience periods of relatively high cooling loads e.g.when the door is opened frequently to remove drinks for customer and/orwhen the fridge is restocked with a large number of yet-to-be-cooledcontainers of beverage.

To cope with the high cooling loads, bottle coolers intended forcommercial premises are equipped with larger vapour compressionrefrigeration systems than usually found in domestic fridges of similarvolume. This makes them less economical to run than domestic fridges ofcomparable size.

Other than because of their size, the larger compressors typically usedin commercial fridges are less efficient than the smaller compressorsused for domestic fridges because the larger market for domestic fridgeshas driven development for greater efficiency in the compressors used.

In addition, the overall efficiency of bottle coolers is compromised bythe need to use materials of relatively low thermal insulatingproperties to make the door see-through/transparent. This issue isparticularly acute with display cabinets intended to operate with anopen front (i.e. no door) during trading hours as are common in shopsfor the sale of refrigerated/frozen goods.

CA2103978 relates to a system having two refrigeration chambers, one fora fridge and the other for a freezer, each having an evaporator. Acontrol device is used to direct refrigerant flow between theevaporators to control the temperature of the chambers. A phase changematerial may be used in conjunction with either evaporator.

DE202006010757 relates to a refrigerated display cabinet having a phasechange material on an inside wall to act as a thermal accumulator.

The present invention was conceived with the aim to increase theefficiency of refrigeration display cabinets, though the invention isthought to be of benefit to any refrigeration device that experiencesperiodic large load variations.

In accordance with the invention, a refrigerator is provided having acooling chamber for containing an object to be cooled, the refrigeratorcomprising: a thermal store comprising a phase change material; a vapourcompression refrigeration system including a first evaporator forcooling the cooling chamber and a second evaporator for cooling thephase change material; and means to control the flow of refrigerant tothe first and second evaporators depending on the cooling load on therefrigerator, wherein, when the refrigerator is subject to a relativelylow cooling load, refrigerant flows to the second evaporator to cool thephase change material and, when the refrigerator is subject to arelatively high cooling load, refrigerant flows to the first evaporatorsuch that increased cooling is provided to the cooling chamber by thefirst evaporator and the phase change material.

When the refrigerator is subject to a relatively low cooling load, it isenvisaged that refrigerant may flow substantially only to the secondevaporator to cool the phase change material, since the phase changematerial and the second evaporator will provide some cooling effect.However, in a preferred embodiment refrigerant flows to both the firstand second evaporators when the refrigerator is subject to a relativelylow cooling load.

When the refrigerator is subject to a relatively high cooling load, itis preferred to maximise the cooling effect provided by the firstevaporator by directing the refrigerant substantially only to the firstevaporator.

By directing refrigerant through the second evaporator during periodswhen the cooling load is low, the spare capacity in the system can beused to cool the phase change material (PCM) into the lower energystate, e.g. from a gas to liquid or a liquid to a solid.

When the refrigerator experiences a high cooling load, both the firstevaporator and the thermal store can be used simultaneously(sequentially) to cool the air. As the first evaporator and thermalstore are separate, the cooling surface area is increased enablingfaster cooling. Since the PCM is at least partially if not completely inthe lower energy state from the previous low cooling load period, therefrigerant flow can be directed (primarily or completely) to the firstevaporator in favour of the second evaporator, so that the cooling ofthe air provided by the first evaporator is increased. Even though therefrigerant flow to the second evaporator is restricted or turned off,the PCM in its lower energy state will continue to cool the air as itgradually transitions to the higher energy state. Therefore, therefrigerant can be used primarily to cool the air rather than the PCMduring periods of peak loading.

Through the control of the refrigerant in this way the efficiency of thesystem is improved. The thermal store provides an effect analogous tospreading the cooling load over a longer period. This allows the systemto use a smaller more efficient compressor with minimal if any reductionin the effectiveness of the refrigerator.

The thermal store may also enable the refrigerator to continue coolingin the event of temporary power failure.

The state of the cooling load is typically identified by sensing atemperature difference between the actual temperature and the desiredtemperature.

To improve the cooling rate of air within the cooling chamber, it ispreferred that there comprises means to circulate air in the coolingchamber past a cooling surface of the thermal store, and preferably alsopast the first evaporator. Forced air cooling is considered to beparticularly favourable for display refrigerators where cooling throughconvection or conduction may not be practical to provide the coolingrate needed to cope with the higher thermal load. Preferably, the meansto circulate air comprises a fan. It is preferred that the air iscirculated out of the cooling chamber, past the cooling surface of thethermal store, past the first evaporator and back into the coolingchamber.

The refrigerator may comprise a duct which is interposed between a wallof the cooling chamber and an external insulating wall of therefrigerator. The thermal store may be mounted within the duct. Toprovide the greatest surface area, it is preferred that the thermalstore is mounted within the duct so that two opposing external sides ofthe thermal store are exposed to air flowing through the duct. In otherwords, air passes on both sides of the thermal store. Preferably, thethermal store is elongate with its elongate axis parallel to the axis ofthe duct, so that the thermal store presents a large proportion of itssurface area to the passing air.

Preferably the first evaporator is positioned downstream of the thermalstore so that the full cooling effect of the PCM can be used duringperiods of high cooling load, to cool the air before it passes over thefirst evaporator.

The vapour compression refrigeration system preferably further comprisesa compressor, a condenser, at least one expansion device, a firstpathway through which refrigerant flows to the first evaporator and backto the compressor, and a second pathway through which refrigerant flowsto the second evaporator and back to the compressor. In a preferredembodiment, the second pathway downstream of the second evaporatormerges with the first pathway upstream of the first evaporator.

The means to control the flow of refrigerant between the first andsecond evaporators may act to route refrigerant to both evaporators (inequal or unequal rates) or completely or substantially to one or theother

Preferably, the means to control the flow of refrigerant to the firstand second evaporators depending on the cooling load comprises a valveand a controller. Preferably, the controller controls the position ofthe valve depending upon the cooling load. As mentioned above, thecooling load is preferably determined by determining the differencebetween the temperature of the air within the cooling chamber and/orduct and the desired temperature. The temperature of the air within thecooling chamber and/or duct is preferably determined by a temperaturesensor.

Preferably, the controller also controls the position of the valvedepending on the relative proportions of the two phases of the PCM.

In a preferred embodiment, the valve has only two positions, a firstposition in which the refrigerant is directed to both evaporators and asecond position in which the refrigerant is directed only to the firstevaporator. The valve is preferably a bistable valve. This allows asimplified control system.

The controller preferably also controls the operation of the compressor,and/or the operation and/or speed of the means to circulate airmentioned above.

It is preferred that the PCM comprises water as a primary constituent,which may also include one or more solutes to adjust the freezing point.

The second evaporator may be arranged to be partially or completelylocated within the PCM, so that the PCM is cooled from the insideoutwards. It is preferred that the thermal store comprises means toidentify the extent to which the PCM has changed state. This informationcan be used by the controller to control the flow of refrigerant to thesecond evaporator and/or to control the compressor.

In an alternative aspect, the invention provides a refrigerator having acooling chamber for holding an object to be cooled, comprising: athermal store including a phase change material to cool the coolingchamber during a period when the refrigerator is subject to relativelyhigh cooling load; a vapour compression refrigeration system comprising:a first evaporator through which a refrigerant flows to cool the coolingchamber; and a second evaporator through which refrigerant flows to coolthe phase change material when the refrigerator is subject to arelatively low cooling load; and means to control the flow ofrefrigerant to the first and second evaporators depending on the coolingload on the refrigerator.

The invention will now be described by example with reference to thefollowing figures in which:

FIG. 1 is a part schematic side sectional view of a refrigerationdisplay cabinet in accordance with the invention;

FIG. 2 is a schematic drawing of a refrigeration circuit in accordancewith the invention; and

FIG. 3 is a part schematic side view illustrating the thermal store.

With reference to FIG. 1, a refrigeration display cabinet comprises athermally insulating casing 1 with a glass door 2. The casing ispreferably made from vacuum-formed insulating panel combined withhigh-density polyurethane for structural rigidity. The glass door may bedouble-glazed or preferably triple-glazed. Krypton gas may be providedbetween the glass plates to increase insulation. The cabinet rests on abase which holds components of a vapour compression refrigerationincluding a compressor 3, a condenser 4, and a fan 5 associated with thecondenser 4.

The cabinet 1 has a compartment 6 in which products to be held arecooled. Lighting may be provided for the compartment 6, which ispreferably energy-efficient LED lighting. The lighting power supply ispreferably located outside the compartment 6. In order to decrease theappliance heat load still further, the LED light source may also belocated outside the compartment 6 and the light guided in to thecompartment 6 by appropriate means, such as light guides, fibre optics,aerogels, etc.

Air is drawn from compartment 6 into ducting 7 by a fan 7A in order tobe cooled. The ducting 7 is defined in part at least by a gap betweeninternal walls 6A which form the compartment 6 and the inner wall of theinsulating casing 1. The base of the compartment 6 may be defined by afirst evaporator 10 (referred to below) or casing thereof.

FIGS. 1 and 2 show the refrigerant circuit. Condensed refrigerant fromthe condenser 4 can flow optionally along one of two path ways back tothe compressor 3. A first pathway 8 carries refrigerant through a firstexpansion device 9A and first evaporator 10, typically a fin and tubeevaporator. The second pathway 11 carries refrigerant through a secondexpansion device 9B and a second evaporator 12 which is embedded withina thermal heat storage unit 13 holding a phase change material (PCM) 14,in this case water.

Flow of refrigerant is controlled by a valve 15 downstream of thecondenser 4. The position of the valve 15 is controlled by a controller16, which is also used to control compressor 3, condenser fan 5 andducting fan 7A.

As shown in FIGS. 1 and 2, the system is arranged such that refrigerantthat has flowed from the second evaporator 12 then flows back to thecompressor 3 via the first evaporator 10. Other arrangements arepossible, including two separate pathways which merge up-stream of thecompressor 3.

Returning to FIG. 1, both the first evaporator 10 and the thermalstorage unit 13 are mounted so that the air circulating through theducting 7 passes across cooling surfaces of the thermal storage unit 13and the first evaporator 10 to cool the air.

A temperature sensor 17 senses the temperature of the air coming out ofthe compartment 6 and provides a corresponding signal to controller 16.

The first evaporator 10 is positioned downstream of the thermal store13, so that the warmest air passes over the thermal store 13 increasingthermal transfer from the PCM 14 during periods of high cooling loading.

As seen in FIG. 3, the second evaporator 12 is embedded within thethermal store 13 so that freezing of the PCM 14 occurs first in acentral region 14A, which is primarily ice, outside of which are outerregions 14C formed primarily of water. The extent to which the PCM 14has frozen/melted is detected through registering the position of theice/water interface 14B. This is achieved by a sensor(s) 18 whichmeasures the electrical conductivity of the PCM 14 at points between theouter wall of the thermal store 13 and the evaporator 12. The signalsfrom the sensor(s) 18 are received by the controller 16. Sucharrangements are known in the art of thermal stores incorporating PCMs.

To maximise the cooling surface presented by the thermal store 13 to theair within duct 7, the thermal store 13 is spaced from both wall 6A andcasing 1 so that air can pass across either side of it.

Returning to FIGS. 1 and 2, the operation of the refrigerator will nowbe described. When the refrigerator is operating in a steady state mode,i.e. the temperature of the air flowing out of the compartment is at ornear the desired temperature, the controller 16 operates valve 15 sothat refrigerant is pumped along the second pathway 11 through thesecond evaporator 12 so as to cool and freeze the PCM 14 within thethermal store 13. Once the PCM 14 has frozen as determined by sensor 18,the controller 16 can cause the compressor 3 and condenser fan 5 to turnoff/slow down to save energy. Typically, such compressors are either onor off, but a reduction in speed may be possible in some cases.

By applying such control, the state of freezing of the PCM 14 duringsteady state operation can be controlled for example between completelyfrozen and 20% melted to ensure sufficient PCM 14 is frozen to provideadditional cooling during the next period of high cooling load. Ifduring steady state operation it is determined that the PCM 14 issufficiently frozen, the controller 16 can cause the compressor 3 toturn off or reduce in speed to reduce energy consumption.

In addition, the controller may control the operation or speed of fan 7Aduring this steady state, as a further way of adjusting and controllingthe compartment air temperature.

In a specific embodiment of steady-state operation, refrigerant flowsthrough both evaporators and the product temperature is controlled byregulating the air temperature leaving the compartment. The airtemperature is controlled by adjusting the speed of the fan 7A andswitching the compressor 3 and condenser fan 5 on and off. On top ofthis, if the quantity of frozen PCM as measured by sensor 18 drops belowa threshold value, the compressor 3 and condenser fan 5 are activated.If the air temperature becomes too low, the speed of fan 7A is reduced.Once the PCM reaches or nears 100% frozen, the compressor/fan aredeactivated and the thermal store/PCM cools the air. As air temperatureincreases, the fan speed is increased until another thresholdtemperature is reached and the compressor/fan are then activated.

During periods of relatively high thermal loading, as determined bysensor 17 detecting that the temperate of air from the compartment 6 isabove the desired temperature (perhaps by more than an accepted rangefrom the desired temperature), the controller 16 adjusts valve 15 sothat refrigerant is preferentially directed to the first evaporator 10.This provides the first evaporator 10 with greater cooling power to coolthe circulating air. The cooling load on the first evaporator 10 is alsolessened by the cooling effect of the thermal storage unit 13 on the airwhich first passes across it.

Once it is sensed that the temperature has fallen to or around thedesired temperature, the controller 16 will operate valve 15 to causethe or a portion of the flow of refrigerant to be directed through thesecond evaporator 12 to refreeze the PCM 14, and ultimately steady-stateconditions will be reached.

It will appreciate that there are numerous possible variations to theembodiments described above without departing from the scope of theinvention, which is defined by the claims. For example, the refrigeratormay comprise more than two evaporators. The temperature sensor mayinstead be mounted in the compartment 6.

As mentioned above, the thermal store may enable the refrigerator tocontinue cooling in the event of a temporary power failure. However, abattery may also be provided to run the vapour compression system in theevent of power failure.

1. A refrigerator comprising: a cooling chamber configured and arrangedto contain an object to be cooled; a thermal store including a phasechange material; a vapor compression refrigeration system including afirst evaporator for cooling the cooling chamber and a second evaporatorfor cooling the phase change material; and a valve and a valve positioncontroller circuit configured and arranged to control flow ofrefrigerant to the first and second evaporators depending on the coolingload on the refrigerator, by when the refrigerator is subject to arelatively low cooling load, controlling a position of the valve withthe valve position controller circuit to flow refrigerant to the secondevaporator to cool the phase change material and, when the refrigeratoris subject to a relatively high cooling load, controlling the positionof the valve with the valve position controller circuit to flowrefrigerant to the first evaporator such that increased cooling isprovided to the cooling chamber by the first evaporator and the phasechange material, relative to cooling provided when the refrigerator issubject to the relatively low cooling load.
 2. The refrigerator of claim1, wherein the valve and valve position controller circuit areconfigured and arranged to flow refrigerant to both the first and secondevaporators when the refrigerator is subject to the relatively lowcooling load.
 3. The refrigerator of claim 1, wherein the valve andvalve position controller circuit are configured and arranged to flowrefrigerant substantially only to the first evaporator when therefrigerator is subject to a relatively high cooling load.
 4. Therefrigerator of claim 1, wherein the phase change material exhibits atleast two phases, and the valve position controller circuit isconfigured and arranged to control the position of the valve based onrelative proportions of the phase change material in each of the atleast two phases.
 5. The refrigerator of claim 1, wherein the valve is abistable valve.
 6. The refrigerator of claim 1, further including atemperature sensor configured and arranged to determine the temperatureof the cooling chamber, wherein the controller circuit is configured andarranged with the temperature sensor to control the position of thevalve based on temperature indicated by the temperature sensor.
 7. Therefrigerator of claim 1, further including sensor configured andarranged to determine the relative proportions of the phases of thephase change material, and wherein the valve position controller circuitis configured and arranged to control the valve position based on therelative proportions of the phases of the phase change materialindicated via the sensor.
 8. The refrigerator of claim 1, wherein thevapor compression refrigeration system includes a compressor, andwherein the valve position controller circuit is configured and arrangedto control the amount of cooling provided to the cooling chamber bycontrolling the compressor.
 9. The refrigerator of claim 1, wherein therefrigerator further comprises a fan configured and arranged tocirculate air from the cooling chamber to the thermal store and thefirst evaporator for cooling, and the valve position controller circuitis configured and arranged with the fan to control the amount of coolingprovided to the cooling chamber by controlling the operation or speed ofthe fan.
 10. The refrigerator of claim 1, wherein the thermal storeincludes two opposing cooling surfaces configured and arranged to flowair over either side of the thermal store to pass across both coolingsurfaces.
 11. The refrigerator of claim 10, wherein the first evaporatoris positioned downstream of the thermal store.
 12. The refrigerator ofclaim 1, wherein the second evaporator is embedded within the thermalstore.
 13. The refrigerator of claim 1, wherein the valve and valveposition controller circuit are configured and arranged to flow therefrigerant substantially only to the first evaporator in response to atemperature within the refrigerator exceeding a desired temperature and,in response to the temperature dropping back to the desired temperature,freezing the phase change material by flowing refrigerant to the secondevaporator.
 14. A method for use with a refrigerator having a coolingchamber configured and arranged to contain an object to be cooled, athermal store including a phase change material, a vapor compressionrefrigeration system including a first evaporator for cooling thecooling chamber and a second evaporator for cooling the phase changematerial, the method comprising: when the refrigerator is subject to arelatively low cooling load, cooling the phase change material byflowing refrigerant to the second evaporator, and when the refrigeratoris subject to a relatively high cooling load, providing increasedcooling to the cooling chamber from the first evaporator and the phasechange material by flowing refrigerant to the first evaporator.
 15. Themethod of claim 14, wherein flowing refrigerant to the second evaporatorand flowing refrigerant to the first evaporator include using a valveand a valve position controller circuit to control flow of refrigerantto the first and second evaporators based on the cooling load.
 16. Themethod of claim 14, further including flowing refrigerant to the firstevaporator when the refrigerator is subject to the relatively lowcooling load.
 17. The method of claim 14, wherein providing increasedcooling to the cooling chamber from the first evaporator includesflowing refrigerant substantially only to the first evaporator.
 18. Themethod of claim 14, wherein flowing refrigerant to the second evaporatorincludes flowing the refrigerant based on relative proportions of thephase change material in respective phases.
 19. The method of claim 14,wherein providing the increased cooling to the cooling chamber includesflowing air over the thermal store and the first evaporator.
 20. Themethod of claim 14, wherein providing the increased cooling includesflowing the refrigerant substantially only to the first evaporator inresponse to a temperature within the refrigerator exceeding a desiredtemperature, further including, in response to the temperature droppingback to the desired temperature, freezing the phase change material byflowing refrigerant to the second evaporator.